The traditional approach to accident control in a nuclear power plant has been to develop many "emergency" procedures, each based on a postulated event, such as loss of main feedwater. The power plant operator was required to study this event and memorize its symptoms and the action required to remedy same. If a loss of feedwater occurred, the operator was expected to recognize this condition, perform the appropriate immediate actions, and then use the event-oriented loss-of-feedwater procedure for determining follow-up actions. The foregoing procedure has several inherent disadvantages:
1. The power plant operator must promptly and correctly diagnose the initiating event. He accomplishes this mentally, based on training and prior experience. As a result of this instant evaluation, he then selects the event-oriented procedure that best fits his diagnosis. For example, if his evaluation indicates that a small steam line break has occurred in the reactor building when, in actuality, a small loss of coolant has occurred in the building, he would implement the wrong event-oriented procedure. Eventually he will recognize his error, however, by that time, the condition may have substantially worsened.
2. Procedures must be written to cover every conceivable initiating event. If the power plant operator correctly diagnoses an initiating event and no procedure has been developed to cover that event, his actions will be based on his level of experience.
3. If more than one event contributes to the transient, the operator will find that he has to work on two or more procedures simultaneously. These procedures may conflict and the operator must establish a priority with respect thereto. It is possible to write a procedure combining two events, however, if more failures are considered, the number of failure combinations, along with their initiating events, becomes extremely large. Even if such procedures could be established, the large number of procedures would definitely have an adverse effect on the operator's ability to select the proper procedure.
4. Because of the foregoing limitations, most operators are likely to use no specific procedure. Instead, they will use training, experience, intuition, etc. to bring the plant under control. They will then select what they believe is the proper procedure to remedy the condition and will later review the results to determine if their procedure selection was appropriate.
Because of the disadvantages associated with an event-oriented approach to determining and correcting malfunctions in a nuclear reactor installation, it has become desirable to formulate and develop a new approach for promptly identifying and correcting malfunctions in such a system.